WO1997018640A1 - Echo cancellation for modems - Google Patents

Echo cancellation for modems Download PDF

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Publication number
WO1997018640A1
WO1997018640A1 PCT/US1996/018122 US9618122W WO9718640A1 WO 1997018640 A1 WO1997018640 A1 WO 1997018640A1 US 9618122 W US9618122 W US 9618122W WO 9718640 A1 WO9718640 A1 WO 9718640A1
Authority
WO
WIPO (PCT)
Prior art keywords
far
echo
modem
listener
canceler
Prior art date
Application number
PCT/US1996/018122
Other languages
French (fr)
Inventor
William Lewis Betts
Edward Sigmund Zuranski
Original Assignee
Paradyne Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Paradyne Corporation filed Critical Paradyne Corporation
Publication of WO1997018640A1 publication Critical patent/WO1997018640A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B3/00—Line transmission systems
    • H04B3/02—Details
    • H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
    • H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
    • H04B3/237—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers using two adaptive filters, e.g. for near end and for end echo cancelling

Definitions

  • This invention relates generally to improving modem performance and, in particular, to improving modem performance by employing echo cancellation techniques
  • Far listener echo is the multiple echo of a signal originating at a transmitting modem.
  • the signal from the transmitting modem a) reflects off the hybrid in the telephone central office (CO) serving the receiving modem, b) returns back to the hybrid in the CO serving the transmitting modem, c) reflects off the hybrid in the CO serving the transmitting modem back to the hybrid in the CO serving the receiving modem, and d) arrives back at the transmitting modem.
  • CO telephone central office
  • each carrier inserts a 6 dB attenuation in the transmit path of each connection. Such an attenuation reduces the far listener echo.
  • the telephone network does not include the attenuation necessary to effectively reduce far listener echo.
  • far listener echo can degrade the performance of a modem.
  • far listener echo appears as a degraded signal to noise ratio (SNR) of the receiver of the transmitting modem.
  • SNR signal to noise ratio
  • a transmitting modem is equipped with the ability to cancel the far listener echo.
  • Cancellation of the far listener echo is achieved by coupling the signal containing the in-phase, quadrature (also known as X and Y, or real and imaginary, components of the) modem transmit signal to a far echo canceller that is included in the modem in addition to the far echo canceller conventionally found in modems.
  • the bulk delay employed by the additional far echo canceller is arranged to be a delay of two round trips between the transmitting modem and the receiving modem rather than the conventional single round trip delay of the conventional far echo canceller
  • the far listener echo canceller may be implemented at a lower cost than the original far echo canceller
  • the delay of two round trips may be implemented by coupling the output of the one round trip delay used for far echo cancellation to an additional delay of one round trip, in accordance with an aspect of the invention
  • the additional far echo canceller is arranged to double the f-equency offset angle of the phase locked loop of the original far echo canceller to provide twice the coordinate rotation as is used by the original far echo canceller Doing so compensates for the doubling of the frequency offset introduced by the additional round trip that the signal has made through the network without requiring the additional far echo canceller to have its own phase locked loop
  • FIG 1 shows an exemplary embodiment of a system for use b a transmitting modem in canceling far listener echo, in accordance with the principles of the invention
  • FIG 2 shows another embodiment of the invention in which the cost of canceling the far listener echo is reduced by employing additional aspects of the invention Detailed Description
  • FIG 1 shows an exemplary embodiment of a system for use by a transmitting modem in canceling far listener echo, in accordance with the principles of the invention
  • a) far echo canceller 151 is coupled to b) far listener echo canceller 153
  • the structure of far echo canceller 151 and far listener echo canceller 152 are identical Each contains one of a) echo canceller finite impulse response (FLR) filter 101, b) least mean square (LMS) update 102, c) multipliers 103 and 104 for perforrrung coordinate rotation, d) adder 105, e) frequency offset phase locked loop (PLL) 106, f) complex conjugate 107, and g) bulk delay 108
  • the components of far echo canceller 151 are designated by a suffix of "1" while the components of far listener echo canceller 152 are designated by a suffix of "2"
  • bulk delay 108-1 being configured for one-round-t ⁇ p
  • bulk delay 108-2 being configured for a two-round-t ⁇ p delay
  • FIG 2 shows another embodiment of the invention in which the cost of canceling the far listener echo is reduced by employing additional aspects of the invention
  • Identically numbered components of FIG 1 and FIG 2 perform the identical function while similarly numbered components diffe ⁇ ng by se ⁇ es number, l e , 100 or 200, perform similar functions
  • the output bulk delay 108-1 is supplied to bulk delay 208
  • Bulk delay 208 has a delay of one round t ⁇ p, I e , a delay identical to that of bulk 7/18640
  • the output of bulk delay is delayed by two round trips while, advantageously, only enough memory to implement an additional round trip is required
  • the output of bulk delay 208 is supplied to echo canceller finite impulse response (FLR) filter 101-2 as was the identical output of bulk delay 108-2
  • the output of frequency offset phase locked loop 106-1 is applied twice to the signal supplied as an output by far listener echo canceller finite impulse response filter 101-2, first by multiplier 104-2 and then again by multiplier 204
  • Each of multipliers 104-2 and 204 effect a coordinate rotation of their respective inputs by same angle as is effected by 104-1, since they are each have as an input the output of frequency offset phase locked loop 106-1 Consequently, the series connection of multipliers 104-2 and 204 results in the output of multiplier 204 being rotated by twice the amount of rotation as the output of multiplier 104-1
  • the error signal supplied as input to multiplier 103-1 must be rotated twice in order to omit a second phase locked loop Therefore, the one time rotated error signal developed as an output by multiplier 103-1 is supplied as input to multiplier 103-2
  • the output of multiplier 103-2 is twice rotated with respect to the original error signal supplied as an output from adder 105-2

Abstract

To improve the signal to noise ratio of the receiver of a transmitting modem, the transmitting modem is equipped with the ability to cancel the far listener echo. Advantageously, doing so allows higher data rates or reduced error rates. Cancellation of the far listener echo is achieved by coupling the signal containing the in-phase, quadrature (also known as X and Y, or real and imaginary, components of the modem) transmit signal to a far echo canceller (101-2) that is includes in the modem in addition to the far echo canceller (101-1) conventionally found in modems. However, the bulk delay employed by the additional far echo canceller (101-2) is arranged to be a delay of two round trips (108-2) between the transmitting modem and the receiving modem rather than the conventional single round trip delay (108-1) of the conventional far echo canceller (101-1).

Description

ECHO CANCELLATION FOR MODEMS Technical Field
This invention relates generally to improving modem performance and, in particular, to improving modem performance by employing echo cancellation techniques
Background of the Invention
Far listener echo is the multiple echo of a signal originating at a transmitting modem. In particular, the signal from the transmitting modem a) reflects off the hybrid in the telephone central office (CO) serving the receiving modem, b) returns back to the hybrid in the CO serving the transmitting modem, c) reflects off the hybrid in the CO serving the transmitting modem back to the hybrid in the CO serving the receiving modem, and d) arrives back at the transmitting modem.
In the United States telephone network, each carrier inserts a 6 dB attenuation in the transmit path of each connection. Such an attenuation reduces the far listener echo.
Summary of the Invention
However, in many countries, the telephone network does not include the attenuation necessary to effectively reduce far listener echo. In such situation, we have recognized that far listener echo can degrade the performance of a modem. In particular, far listener echo appears as a degraded signal to noise ratio (SNR) of the receiver of the transmitting modem.
In accordance with the principles of the invention, a transmitting modem is equipped with the ability to cancel the far listener echo. Advantageously, doing so results in an improved SNR for the receiver of the transmitting modem, thus allowing higher data rates or reduced error rates. Cancellation of the far listener echo is achieved by coupling the signal containing the in-phase, quadrature (also known as X and Y, or real and imaginary, components of the) modem transmit signal to a far echo canceller that is included in the modem in addition to the far echo canceller conventionally found in modems. However, the bulk delay employed by the additional far echo canceller is arranged to be a delay of two round trips between the transmitting modem and the receiving modem rather than the conventional single round trip delay of the conventional far echo canceller
In accordance with an aspect of the invention, because of inherent losses in telephone networks and the nature of echoes, even when there is no attenuation intentionally introduced by the telephone network the additional far echo canceller need not have the same cancellation ability as the original far echo canceller Advantageously, the far listener echo canceller may be implemented at a lower cost than the original far echo canceller
Additionally, to further save cost, the delay of two round trips may be implemented by coupling the output of the one round trip delay used for far echo cancellation to an additional delay of one round trip, in accordance with an aspect of the invention
To achieve even greater cost savings, in accordance with an aspect of the invention, the additional far echo canceller is arranged to double the f-equency offset angle of the phase locked loop of the original far echo canceller to provide twice the coordinate rotation as is used by the original far echo canceller Doing so compensates for the doubling of the frequency offset introduced by the additional round trip that the signal has made through the network without requiring the additional far echo canceller to have its own phase locked loop
Brief Description of the Drawing In the drawing
FIG 1 shows an exemplary embodiment of a system for use b a transmitting modem in canceling far listener echo, in accordance with the principles of the invention, and
FIG 2 shows another embodiment of the invention in which the cost of canceling the far listener echo is reduced by employing additional aspects of the invention Detailed Description
FIG 1 shows an exemplary embodiment of a system for use by a transmitting modem in canceling far listener echo, in accordance with the principles of the invention
In the embodiment of the invention shown in FIG 1 , a) far echo canceller 151 is coupled to b) far listener echo canceller 153 The structure of far echo canceller 151 and far listener echo canceller 152 are identical Each contains one of a) echo canceller finite impulse response (FLR) filter 101, b) least mean square (LMS) update 102, c) multipliers 103 and 104 for perforrrung coordinate rotation, d) adder 105, e) frequency offset phase locked loop (PLL) 106, f) complex conjugate 107, and g) bulk delay 108 The components of far echo canceller 151 are designated by a suffix of "1" while the components of far listener echo canceller 152 are designated by a suffix of "2" Other than bulk delay 108-1 being configured for one-round-tπp and bulk delay 108-2 being configured for a two-round-tπp delay, in accordance with an aspect of the invention, the components of far echo canceller 151 and far listener echo canceller 152 are identical Additionally, the components of far echo canceller 151 and far listener echo canceller 152 are interconnected in the conventional, well known manner for a far echo canceller, except that where far echo canceller 151 receives as an input to adder 105-1 the digital version of the signal on the two wire telephone line connecting the modem to the central office after cancellation of only the near echo by near echo canceller 1 1 1, far listener echo canceller 152 receives as an input to adder 105-2 the digital version of the signal on the two wire telephone line connecting the modem to the central office after cancellation of both the near echo and the far echo as output from adder 105-1 of far echo canceller 151
FIG 2 shows another embodiment of the invention in which the cost of canceling the far listener echo is reduced by employing additional aspects of the invention Identically numbered components of FIG 1 and FIG 2 perform the identical function while similarly numbered components diffeπng by seπes number, l e , 100 or 200, perform similar functions
In the manner shown, the output bulk delay 108-1 is supplied to bulk delay 208 Bulk delay 208 has a delay of one round tπp, I e , a delay identical to that of bulk 7/18640
4 delay 108 By so concatenating bulk delays 108 and 208, in accordance with an aspect of the invention, the output of bulk delay is delayed by two round trips while, advantageously, only enough memory to implement an additional round trip is required The output of bulk delay 208 is supplied to echo canceller finite impulse response (FLR) filter 101-2 as was the identical output of bulk delay 108-2
To avoid the cost of including two phase locked loops, in the embodiment of the invention shown in FIG 2, the output of frequency offset phase locked loop 106-1 is applied twice to the signal supplied as an output by far listener echo canceller finite impulse response filter 101-2, first by multiplier 104-2 and then again by multiplier 204 Each of multipliers 104-2 and 204 effect a coordinate rotation of their respective inputs by same angle as is effected by 104-1, since they are each have as an input the output of frequency offset phase locked loop 106-1 Consequently, the series connection of multipliers 104-2 and 204 results in the output of multiplier 204 being rotated by twice the amount of rotation as the output of multiplier 104-1 Additionally, the error signal supplied as input to multiplier 103-1 must be rotated twice in order to omit a second phase locked loop Therefore, the one time rotated error signal developed as an output by multiplier 103-1 is supplied as input to multiplier 103-2 Thus, the output of multiplier 103-2 is twice rotated with respect to the original error signal supplied as an output from adder 105-2 Note that, as is well known in the art, the error signal is rotated in the opposite direction from the output of echo canceller finite impulse response filters 101
It is noted that the invention may be implemented in analog or digital circuitry, or in a combination thereof. It is also noted that the invention may be implemented on a digital signal processor The foregoing merely illustrates the principles of the invention It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope

Claims

What is claimed is: 1 A modem comprising means for supplying a transmitted signal; and means for substantially canceling the far listener echo of said transmitted signal
2 The invention as defined in claim 1 wherein said modem includes a far echo canceller and said means for substantially canceling the far listener echo of said transmitted signal includes. a delay of two round trips between said modem and a modem to which said transmitted signal is being supplied, and an additional far echo canceller
3 The invention as defined in claim 2 wherein the echo cancellation ability of said additional far echo canceller is less than the echo cancellation ability of said far echo canceller
4 The invention as defined in claim 2 wherein said far echo canceller contains a phase locked loop for providing the coordinate rotation, and wherein said additional far echo canceller includes means for doubling the frequency offset angle of said phase locked loop, and means for compensating for the two-fold frequency offset of said far listener echo
5 A modem having means for transmitting a signal, comprising a far echo canceller for canceling a far echo of said transmitted signal, and a far listener echo canceller for canceling a far listener echo of said transmitted signal.
6 The invention as defined in claim 5 wherein said far echo canceller and said far listener echo canceller have an identical structure, except that a bulk delay of said far echo canceller is set to equal the round-trip delay between said modem and a remote modem to which said modem is transmitting said signal and that a bulk delay of said far listener echo canceller is set to equal twice the round-trip delay between said modem and said remote modem.
7. The invention as defined in claim 5 wherein said the echo cancellation ability of said far listener echo canceller is less than the echo cancellation ability of said far echo canceller.
8. The invention as defined in claim 5 a bulk delay of said far echo canceller is set to equal the round-trip delay between said modem and a remote modem to which said modem is transmitting said signal and a bulk delay of said far listener echo canceller is also set to equal the round-trip delay between said modem and said remote modem, said bulk delay of said far listener echo canceller being arranged to receive as an input the delayed output of said far echo canceller bulk delay
9. The invention as defined in claim 5 wherein said far echo canceller contains a phase locked loop for providing the coordinate rotation, and wherein said far listener echo canceller includes: means for doubling the frequency offset angle of said phase locked loop; and means for compensating for the two-fold frequency offset of said far listener echo.
AMENDED CLAIMS
[recei ved by the International Bureau on 14 Apri l 1997 ( 14.04.97 ) ; origi na l cl a ims 1 , 3-5 and 7 amended ; origi na l c l a im 2 cancel led ; remai n i ng c l a ims unchanged ( 2 pages ) ] i 1 . A modem comprising: i means for supplying a transmitted signal; means for substantially canceling the far listener echo of said transmitted signal; s a far echo canceler and said means for substantially canceling the far e listener echo of said transmitted signal, said means including a delay of two round trips between said modem and a modem to which said transmitted signal is being s supplied; and
9 an additional far echo canceler.
Cancel claim 2.
l 3. The invention as defined in claim 1 wherein the echo cancellation ability of said additional far echo canceler is less than the echo cancellation ability of said far echo
3 canceler.
i 4. The invention as defined in claim 1 . wherein said far echo canceler
: contains a phase locked loop for providing the coordinate rotation, and wherein said additional far echo canceler includes: -; means for doubling the frequency offset angle of said phase locked loop; and
5 means for compensating for the two-fold frequency offset of said far listener
6 echo.
1 5. A modem having means for transmitting a signal, comprising:
2 a far echo canceler comprising a finite impulse response filter for canceling a far echo of a transmitted signal; and a far listener echo canceler comprising a finite impulse response filter for canceling a far listener echo of said transmitted signal. ; 6. The invention as defined in claim 5, wherein said far echo canceler and said far listener echo canceler have an identical structure, except that a bulk delay of said far echo canceler is set to equal the round-trip delay between said modem and a remote
■ modem to which said modem is transmitting said signal and that a bulk delay of said far listener echo canceler is set to equal twice the round-trip delay between said modem and said remote modem.
i 7. The invention as defined in claim 5, wherein the echo cancellation ability of said far listener echo canceier is less than the echo cancellation ability of said far echo canceler.
8. The invention as defined in ciai 5, wherein a bulk delay of said far echo
: canceler is set to equal the round-trip delay between said modem and a remote modem to which said modem is transmitting said signal and a bulk delay of said far listener echo -; canceler is also set to equal the round-trip delay between said modem and said remote
: modem, said bulk delay of said far listener echo canceler being arranged io receive as an t input the delayed ouφut of said far echo canceler bulk delay.
: 9. The invention as defined in claim 5, wherein said far echo canceler contains a phase locked loop for providing the coordinate rotation, and wherein said far listener echo canceler includes:
4 means for doubling the frequency offset angle of said phase locked loop; and
2 means for compensation for the two-fold frequency offset of said far listener echo.
PCT/US1996/018122 1995-11-14 1996-11-13 Echo cancellation for modems WO1997018640A1 (en)

Applications Claiming Priority (2)

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US08/557,634 US5682378A (en) 1995-11-14 1995-11-14 Echo cancellation for modems
US08/557,634 1995-11-14

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WO1999037038A1 (en) * 1998-01-15 1999-07-22 Siemens Schweiz Ag Method for setting up echo suppression devices in communication links with automatic machines
WO2003067786A1 (en) * 2002-02-04 2003-08-14 Innoace Co., Ltd. Haulingless rf repeater and method thereof
US7150979B2 (en) 2001-02-21 2006-12-19 Clontech Laboratories, Inc. Nucleic acids encoding non-aggregating fluorescent proteins and methods for using the same

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US6192087B1 (en) 1996-11-15 2001-02-20 Conexant Systems, Inc. Method and apparatus for spectral shaping in signal-point limited transmission systems
US6647058B1 (en) * 1997-06-23 2003-11-11 Paradyne Corporation Performance customization system and process for optimizing XDSL performance
US7512149B2 (en) * 2003-04-23 2009-03-31 At & T Intellectual Property Ii, L.P. Bit and power allocation scheme for full-duplex transmission with echo cancellation in multicarrier-based modems
US6678254B1 (en) 1999-05-03 2004-01-13 Nortel Networks Limited Method and communication device for optimizing echo cancellation
US6278785B1 (en) * 1999-09-21 2001-08-21 Acoustic Technologies, Inc. Echo cancelling process with improved phase control
US7023812B1 (en) 2000-02-15 2006-04-04 Siemens Communications, Inc. System and method for improving modem transmission through private branch exchanges, central offices, and other systems
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WO2003067786A1 (en) * 2002-02-04 2003-08-14 Innoace Co., Ltd. Haulingless rf repeater and method thereof

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